Hello,
The specific details for the inputs and outputs if the M1 ECUs can be found in the M1 ECU Hardware PDF at the link below:
https://www.motec.com.au/filedownload.p ... docid=4487The half bridge outputs have internal recirculation diodes.
The low side injector and low side ignition outputs do not have any internal recirculation circuitry.
Whether you need to add an external recirculation diode will depend on the characteristics of the solenoid you are trying to control.
If you have access to a running engine then it would be helpful to use an oscilloscope and determine if there's any significant flyback voltage.